In milling operations, a method of cutting a workpiece by increasing a feed rate (feed distance per tooth) of a milling cutter has been recently introduced.
FIGS. 1 to 3 illustrate variations of feed rates according to various entering angles. As shown in FIGS. 1 to 3, as the entering angles become smaller, a radial load transmitted from a workpiece vertically to an axis of a cutting tool (milling cutter) also becomes smaller accordingly. That is, since the load acting in a vertical direction of the milling cutter axis is reduced as the entering angle becomes smaller, the fluctuation of the milling cutter and the impact caused thereby against cutting edges are reduced, thereby lengthening the service lives of the cutting edges compared to a conventional milling process.
Furthermore, since the feed rate is high when setting the entering angle to be relatively small over a large entering angle, the amount of removed workpiece per rotation of the milling cutter is increased. Thus, the processing efficiency can be raised by processing the workpiece through the milling process of setting the entering angle to be small. As such, a recent milling process adopts the cutting method of setting the entering angle to be small.
FIG. 4 is a perspective view of a conventional cutting insert, which is mounted to a milling cutter. FIG. 5 is a side view of the conventional cutting insert. FIG. 6 illustrates a milling cutter with the conventional cutting insert mounted thereto. As shown in FIGS. 4 and 5, an upper face 15 of the cutting insert 10 is flat, while edges of the upper face 15 function as cutting edges 12. The cutting edge 12 of the cutting insert 10 is generally formed to have a prescribed radius of curvature. Since the cutting edge 12 has a prescribed radius of curvature instead of a straight shape, chips generated while processing a workpiece become thick as they are disposed away from an axis of the milling cutter 11.
As described above, when processing a workpiece at a small entering angle in the milling operations, a large amount of workpiece is removed per rotation of the milling cutter. Accordingly, a high cutting resistance is applied to the cutting insert. In order to reduce the cutting resistance applied to the cutting insert, the cutting insert is mounted to the milling cutter such that the upper face 15 of the cutting insert has a positive rake angle with respect to the axis of the milling cutter 11, as shown in FIG. 6. Further, in order to avoid any friction between a flank face 16 of the cutting insert and a processing surface of the workpiece, a clearance angle is placed between the flank face 16 and the processing surface of the workpiece. Thus, the conventional cutting insert used in the milling operation, wherein the entering angle is set to be small, has a shape such that an area of the upper face 15 is larger than that of a lower face 17, as shown in FIGS. 4 and 5. The cutting insert 10 can be secured to the milling cutter 11 such that the lower face 17 is contacted to a support surface of the milling cutter, wherein a screw is then inserted through a hole 13 formed at a central portion of the cutting insert 10.
Such a conventional cutting insert is permitted to use only one face thereof (i.e., upper face) when cutting the workpiece. The reasons why both faces cannot be used in the conventional cutting insert are as follows. In order to use the lower face of the cutting insert, which is secured to the support surface of the milling cutter, in processing the workpiece, the cutting insert must be turned upside down and the upper face thereof must be secured to the support surface of the milling cutter. In such a case, since the upper face of the cutting insert was already significantly damaged due to collision with chips during processing the workpiece, it cannot be uniformly contacted to the support surface of the milling cutter. Thus, since the cutting insert cannot be firmly secured to the support surface of the milling cutter, considerable fluctuations can be generated during processing the workpiece. Further, since the conventional cutting insert is mounted to the milling cutter so as to form a positive rake angle, the lower face of the cutting insert has a smaller area than the upper face of the cutting insert to prevent the flank face from being contacted to the workpiece (see FIGS. 4 and 5). Thus, even if other cutting edges are formed between the lower face and the flank faces of the cutting insert, when the upper face of the cutting insert is secured to the support surface of the milling cutter, the cutting edge formed at the lower face of the cutting insert cannot be contacted to the processing surface of the workpiece.
When cutting edges have a prescribed radius of curvature similar to those of conventional cutting inserts, generated chips become thicker as they are located away from the axis of the milling cutter. Further, as the chips become thicker, the cutting resistance applied to the cutting insert also becomes higher. Accordingly, a portion of the cutting insert portion, to which the higher cutting resistance is applied, is worn out faster than other portions of the cutting insert. As such, a life span of the cutting insert is significantly reduced.